IPVS: Add genetlink interface implementation

Add the implementation of the new Generic Netlink interface to IPVS and
keep the old set/getsockopt interface for userspace backwards
compatibility.
Signed-off-by: Julius Volz <juliusv@google.com>
Acked-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Simon Horman <horms@verge.net.au>